Fix: cluster default to NONE instead of local if unset; add unittest to test no cluster

[lbesnard]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.51163% with 3 lines in your changes missing coverage. Please review.

Project coverage is 77.52%. Comparing base (c4fa297) to head (277a68b).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
..._optimised/test_generic_zarr_handler_no_cluster.py	96.34%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #102      +/-   ##
==========================================
+ Coverage   76.64%   77.52%   +0.88%     
==========================================
  Files          24       25       +1     
  Lines        2556     2639      +83     
==========================================
+ Hits         1959     2046      +87     
+ Misses        597      593       -4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.